German patent number DE 195 07 401 A1 discloses monitoring soiling of a protective glass through reference measurement on a first optical element and measurement of a soiled second optical element.
During the processing of workpieces with a machine for thermal welding or cutting, (e.g., a laser processing machine) splashes and smoke are produced in the processing region of the workpiece. The splashes and smoke may be deposited on a surface of an optical element that faces the workpiece of the processing head of the machine and may soil the surface of the optical element. The splashes may be burnt into the optical surface. Smoke can be deposited on the optical surface but can be removed from the optical surface to a certain extent. Deposits on or damage to the optical surface can result in an increased absorption of the laser radiation by the optical element. Accordingly, the thermal load of the optical element increases, which results in a noticeable decrease of the laser power available in the processing region. In case of heavy soiling, in particular, through splashes, increased absorption of the laser radiation can result in the destruction of the optical element.
For this reason, it is necessary to detect splashes or smoke on the optical element at an early stage to ensure constant thermal power in the working region of the workpiece and prevent destruction of the optical element by switching off the thermal processing machine in due time or to indicate that cleaning or replacement of the optical element is required.
German patent number DE 195 07 401 C2 discloses the detection and linear evaluation of stray light on the protective glass edge. A reference measured value is generated by measuring the stray light also on an additional transmitting element located in the beam path. In this manner, the influence of the laser power on the measuring signal can be compensated. Disturbances through reverse reflection from the workpiece, welding flares, external light etc., however, disturb the evaluation signal, because these disturbances affect only the protective glass and not the element providing the reference. The linear evaluation may cause problems with regard to overloading or underloading during evaluation. Detection of soiling through smoke in this way may be inaccurate because smoke produces only very little stray light on the protective glass edge.